The disclosure relates generally to a wind turbine and more specifically to a system and method for adjusting the shaft moment set point correction based on environmental conditions resulting in wind turbine load imbalance.
Modern wind turbines operate in a wide range of wind conditions. These wind conditions can be broadly divided into two categories, below rated speeds and above rated speeds. To produce power in these wind conditions, wind turbines may include sophisticated control systems such as pitch controllers and torque controllers. These controllers account for changes in the wind conditions and accompanying changes in wind turbine dynamics. For example, pitch controllers (or pitch systems) generally vary the pitch angle of rotor blades to account for (or adapt to) the changes in wind conditions and turbine dynamics. During below rated wind speeds, wind power may be lower than the rated power output of the wind turbine. In this situation, the pitch controller may attempt to maximize the power output by pitching the rotor blades substantially perpendicular to the wind direction. Alternatively, during above rated wind speeds, wind power may be greater than the rated power output of the wind turbine. Therefore, in this case, the pitch controller may restrain wind energy conversion by pitching the rotor blades such that only a part of the wind energy impinges on the rotor blades. By controlling the pitch angle, the pitch controller/system thus controls the velocity of the rotor blades and in turn the energy generated by the wind turbine.
Along with maintaining rotor velocity, pitch controllers may also be employed to reduce tower oscillations. Tower oscillations or vibrations occur due to various disturbances, such as turbulence, inefficient damping, or transition between the two wind conditions. Moreover, the tower may vibrate along any degree of freedom. For example, the tower may vibrate in a fore-aft direction (commonly referred to as tower nodding), in a side-to-side direction (commonly referred to as tower naying), or along its longitudinal axis (commonly referred to as torsional vibration).
Tower nodding is usually caused by aerodynamic thrust and rotation of the rotor blades. Every time a rotor blade passes in front of the tower, the thrust of the wind impinging on the tower decreases. Such continuous variation in wind force may induce oscillations in the tower. Moreover, if the rotor velocity is such that a rotor blade passes over the tower each time the tower is in one of its extreme positions (forward or backward), the tower oscillations may be amplified. Typically, the oscillations in the fore-aft direction are automatically minimized due to aerodynamic damping. Aerodynamic damping relies on the fact that the top of the tower constantly oscillates in the fore-aft direction. When the top of the tower moves upwind (or forward), the rotor thrust is increased. This increase in rotor thrust pushes the tower back downwind. The downwind push in turn aids in dampening the tower oscillations. Similarly, when the top of the tower moves downwind, the rotor thrust may be decreased. This decrease in rotor thrust pushes the tower back upwind. The upwind push also aids in dampening the tower oscillations.
Typically, the pitch controller utilizes two separate control loops for the two functions, controlling the rotor velocity and reducing the tower oscillations. A rotor velocity control loop is employed to determine a pitch angle to control rotor velocity and a tower-damping control loop is used to compute a pitch angle to reduce tower oscillations. Often, these feedback loops operate relatively independently of each other. For example, the rotor velocity control loop may determine the pitch angle based on rotor velocity, wind speed, and current pitch angle. The tower-damping control loop, on the other hand, may determine the pitch angle based on tower deflection, tower top velocity, tower top acceleration, current pitch angle, and wind speed. Because of this independence, the currently available rotor velocity control loops may compute a pitch angle to maintain rotor speed that may disadvantageously induce tower oscillations instead of reducing them. Moreover, these rotor velocity control loops may cause energy amplification in the rotor near tower resonance frequencies. Such amplification may increase oscillations in the tower and increase the fatigue load placed on the wind turbine. Over time, such fatigue loads may reduce the life of wind turbine parts and increase the expenses associated with wind turbines.
Gravity tends to bend the rotor down. Positive wind shear tends to bend the rotor up, and usually is present (high cycle fatigue) and increases with increasing wind speed. At some operating points wind shear can equal the gravitational force but is opposite in direction and cancels the bending moment from gravity if not corrected. The thrust component also helps to combat the gravity load, since many wind turbines have a rotor tilt angle in the machine head, the rotor naturally tries to pick the rotor up and this bending moment also increases with wind speed. The bending moment is easily determined due to the geometry of the system and the thrust estimation. The bending moment can be accounted for when the amount of shear load is identified. For “standard” conditions (positive wind shear), the moment due to wind shear and thrust/tilt are complimentary and oppose gravity. There are some conditions “negative shear” coupled with the right wind speed so that achieve a balanced aerodynamic load, the nose down wind shear moment is equal and opposite to the nose up thrust/tilt moment so that all that remains in the moment due to gravity (probably seen only rarely).
Shear applies forces to the blades and hub and positive shear essentially transfers bending forces from the blades to the hub and shaft. The wind shear applies an asymmetric load across the rotor which results in a bending moment being transferred to the hub/shaft. Positive shear creates a nose-up bending moment and opposes gravity while negative shear creates a nose-down moment and compliments gravity. There is a limit on the amount of pitching the turbine is able to do to either overcome blade/hub forces or to compensate for gravity. When the shear controller is in a limit, it is also not able to respond to additional steady state or dynamic (relatively fast changes in shear). Being able to adjust for dynamic conditions can reduce fatigue and other dynamic stresses on the shaft and blade/hub assembly. Since the pitch system has limited capabilities, being able to balance the amount of compensation that is being done for both gravity and shear maintains a balance between the pitch system demand and the machine fatigue and ensure the wind turbine is always operating within a target zone (until the environment demands more than the machine can handle) while also constructing a balance between the amount of bandwidth that is being used for the steady state vs. dynamic compensation portions.
Imbalance load limitations such as environmental conditions including shear, gravity, turbulence, wind miss-alignments, etc., result in environmentally induced rotor imbalance. Other forms of rotor imbalance are self-induced due to control, manufacturing, and installation variation.